1. Field of the Invention
This invention relates to an optoelectronic oscillator and a pulse generator, and more particularly to an especially preferred optoelectronic oscillator that can oscillate with a high frequency as an oscillator to generate a clock signal of CPU (Central Processing Unit) and a pulse generator having the optoelectronic oscillators.
2. Description of the Related Art
The clock frequency of the computer in recent years is high-speed such as about several GHz and it is important to accurately synchronize chips in order to have CPU work without malfunction. In order to synchronize accurately, decreasing an electromagnetic noise of electrical wiring and decreasing a timing jitter of clock generator as signal source are specifically required. Regarding decreasing an electromagnetic noise, recently, there is tremendous amount of research on optical interconnection replacing conventional electrical wiring with optical wiring such as a semiconductor optical waveguide. However, there are few proposals of the high purity clock generator for a high-speed optical interconnection decreasing a timing jitter.
For instance, the timing jitter is limited to 35 ps in the computer system whose clock frequency is 2 GHz. In addition, further reduction of the timing jitter is required in higher clock frequency, but it is not easy to achieve. This is because the clock signal is generated by multiplying an electric signal of about 10 MHz which is generally generated by a liquid crystal oscillator and in that case the electric noise generated by a multiplier decides the lower limit of the timing jitter. In the future, a clock frequency of a computer is thought to exceed 10 GHz, and in that case a high-purity clock generator with the lower timing jitter is required to maintain CPU performance accurately.
Recently, there has been a tremendous amount of research on an OEO (Optoelectronic Oscillator), as shown in FIG. 9, as a high purity clock generator or a microwave oscillator with a low timing jitter, mainly in USA and Russia. The clock generator and the microwave oscillator mean an OEO here. An OEO 10E is the thing to connect to a semiconductor laser 11, an optical fiber 21, a photodetector 13, an amplifier 14 for an electric signal and a filter 22 in a loop shape, and oscillate it with a specific frequency. The generated microwave is taken out through an RF port 23 combining an external terminal. Thus, the OEO 10E uses an optical delay circuit given by optical pulse that propagates in the optical fiber 21 for a part of a cavity. Therefore, the influence of electromagnetic noise on the oscillator becomes relatively low, and it can generate high purity microwave and optical pulse with an especially low time jitter.
In general, an oscillation frequency of an OEO is the frequency with the highest gain of frequency response function of the entire circuit among integral multiples of a fundamental oscillation frequency f0 given by following equation.
                    (                  Equation          ⁢                                          ⁢          1                )                                                                      f          0                =                  1                      (                                                            l                  opt                                                  υ                  g                                            +                                                l                  elec                                                  υ                  M                                                      )                                              (        1        )            
In the equation (1), lopt and lelec show lengths (m) of an optical waveguide and an electric circuit, respectively. Furthermore, νg and νm are group velocity of optical pulse and phase velocity of microwave respectively. Especially, group velocity ug is expressed in the following equation.(Equation 2)νg=c/ng  (2)
Here, ng is a group refractive index of the optical delay line and c is speed of light in the free space.
Understandably from the equation (1), the OEO can oscillate directly with desired frequency by adjusting a length of the fiber or transfer function of a filter (an electrical filter or an optical filter). Therefore, an electric circuit which multiplies the clock frequency such as a conventional liquid crystal-oscillator becomes unnecessary. Therefore, a timing jitter of the OEO 10E can be suppressed to about 100 fs for instance at a clock frequency of 10 GHz, and have an advantage that the timing jitter can be reduced by two digits or more compared with a clock generator based on a conventional liquid crystal oscillator.
Because of the above-mentioned advantage, a wide application range of OEO such as an atomic clock combining gas cell which mixes atoms such as cesium (Cs) and rubidium (Rb), a laser source of optical communication and a clock extraction is considered and researches are moving forward. The optoelectronic oscillator is described in patent documents 1 to 5 and non-patent documents 1 to 6.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-535853    Patent Document 2: U.S. provisional application No. 60/117,568    Patent Document 3: U.S. provisional application No. 60/117,452    Patent Document 4: U.S. provisional application No. 60/117,451    Patent Document 5: Japanese Patent Application Laid-Open (kokai) No. H08-18139    Non-Patent Document 1: T. L. Paoli and J. E. Ripper, “Frequency stabilization and narrowing of optical pulses from CW GaAs injection lasers,” IEEE J. Quantum Electron., vol. 6, no. 6, pp. 335-338, June 1970.    Non-Patent Document 2: T. C. Damen and M. A. Duguay, “Optoelectronic regenerative pulser,” Electron. Let., vol. 16, no. 5, pp. 166-167, February 1980.    Non-Patent Document 3: M. Nakazawa, M. Tokuda, and N. Uchida, “Self-sustained intensity oscillation of a laser diode introduced by a delayed electrical feedback using an optical fiber and an electrical amplifier,” Appl. Phys. Lett., vol. 39, no. 5, pp. 379-381, September 1981    Non-Patent Document 4: X. S. Yao and L. Maleki, “Optoelectronic microwave oscillator”, J. Opt. Soc. Am. B, vol. 13, no. 8, pp. 1725-1735, August 1996.    Non-Patent Document 5: S. Mino et al., “Planar lightwave circuit platform with coplanar waveguide for opto-electronic hybrid integration”, J. Lightwave Technol., vol. 13, No. 12, pp. 2320-232, December 1995.    Non-Patent Document 6: M. Yoshida et al., “Mode-Hop-Free, Optical Frequency Tunable 40-GHz Mode-Locked Fiber Laser”, IEEE J. Quantum Electron., vol. 43, No. 8, pp. 704-708, August 2007.
A lot of conventional OEOs use an optical fiber as an optical delay circuit, and it is difficult to integrate compactly due to the bending loss of the optical fiber. Therefore, it has been thought that it is difficult to use an OEO to generate clock signals for CPU. Furthermore, because the optical fiber itself changes its length and refractive index by temperature change and mechanical vibration and an effective optical length changes, a control circuit for temperature control and adjustment of the fiber length is required for its prevention and compensation of the change, this fact also make difficult to downsize an OEO. In addition, design and formation of a semiconductor laser, a photodetector, an amplifier and an electric filter etc., other than an optical fiber, are individually done, and integration on the same substrate is obstructed.